- It must be Zero when the two items are identical.
- It must be constrained by the Triangle Inequality).
- It must be a Positive Number if the two things are not the same things, because 2*Dist(x,y) = Dist(x,y) + Dist(y,x) ≥ Dist(x,x) = 0.
- It can be a Natural Number (for example for the L1 Norm Distance Function - city block distance).
- See: Similarity Score.